Lignin is an aromatic polymer found in all vascular plants. It is currently produced as a co-product of paper pulp and burned for its fuel value. When petroleum feedstock was cheap, lignin was not worth recovering as a chemical feedstock. However, as petroleum feedstocks become scarcer and more expensive, and an ever increasing percentage comes from foreign sources, more consideration is being given to using lignin, which is a domestic renewable feedstock, as a source of mono-aromatics products, particularly phenol.
Lignin comprises roughly 25% of lignocellulosic biomass, which includes all vascular plants. Lignin gives plants rigidity and protection and helps regulate the permeation of water. It is formed within the plant primarily by the dehydrogenative polymerization of three precursors: trans-coniferyl, trans-sinapyl and trans-p-coumaryl alcohol. The relative amounts of each component and their linkages vary with wood types. Trans-coniferyl alcohol is always the major component, and hardwood lignins contain more trans-sinapyl structures.
In wood pulping via the kraft process, cellulose, hemicellulose and lignin are separated by dissolving hemicellulose and lignin in an alkaline sodium sulfate/sulfide solution at elevated temperature and pressure, yielding the so-called black liquor. Wood lignin is considerably changed by kraft cooking. Condensations involving formaldehyde or active beta-carbonyl derivatives leads to methylene linked aromatic rings and an increase in molecular weight. At the same time, alkaline cleavage of phenolic ethers occurs. As a result of the alkaline cleavage, phenolic hydroxyls increase from 0.3 per monomer in the protollignin to almost 1.0 in the dissolved lignin.
When sufficiently depolymerized, the lignin phenolate form dissolves in the black liquor. The phenolic groups can be liberated by acidification with carbon dioxide obtained from stack gases. This causes the lignin to precipitate, aided by the salting-out effect of the sodium salts of the black liquor. Lignin is filtered at 60.degree.-80.degree. C. Further purification by solution in dilute alkali and reprecipitation with sulfuric acid gives a kraft lignin, such as that used as feedstock in our invention (Table 1). Kraft lignin has a typical average molecular weight of about 3500, which indicates that the average softwood kraft lignin is comprised of about 20 starting units.
TABLE 1 ______________________________________ ANALYSIS OF KRAFT LIGNIN CHARGE STOCK (W %) ______________________________________ Ash by Combustion 1.40 Water 1.02 Carbon (organic) 64.89 Carbon (Na.sub.2 CO.sub.3) 0.08 Hydrogen (ex. H.sub.2 O) 5.67 Sulfur (Na.sub.2 SO.sub.4) 0.16 Sulfur (organic) 1.19 Oxygen (Na.sub.2 CO.sub.3 + Na.sub.2 SO.sub.4) 0.64 Oxygen (organic sulfur) 0.60 Oxygen (organic) 25.23 Sodium (Na.sub.2 CO.sub.3 + Na.sub.2 SO.sub.4) 0.52 100.00 Organic, W % Carbon 64.89 Hydrogen 5.67 Oxygen 25.23 95.79 ______________________________________
U.S. Pat. No. 2,328,749 to Sherrard discloses an early batch-type process for catalytic hydrogenation of wood material to remove lignin and make cellulose. Also, U.S. Pat. No. 2,947,739 to Gaslini discloses a process for hydrogenation of ligno-cellulosic materials using soluble metal carbonyl catalyst at relatively low temperature and pressure conditions. U.S. Pat. No. 2,991,314 to Giesen discloses a continuous non-catalytic process for cleavage of lignin to produce phenols at temperatures above 300.degree. C. and pressure above 350 atm. to yield distillable lignin products. Also, hydrogenation of lignin using the Noguchi batch-type process developed in Japan is disclosed by Goheen in Adv. in Chem. Ser. No. 59,226 (1966). However, further process improvements are needed to develop a continuous process for hydrocracking lignin-containing feedstocks and produce high yields of useful phenol and benzene products along with some distillable oils.